I also agree with every word of Penny's advice. If you are not familliar with Europe or the ports you'll be visiting, venturing out on your own can be unwise and frustrating even if you read all about the ways others more familliar with the train system and its ease and low cost take advantage of. Toting your luggage from some of the airport locations to the sometimes distant pier locations can present challenges many with limited mobility or other factors could be impossible or at a minimum a great challenge. Knowing which transfer tickets or trains needed for the second leg can get you lost or scrambling for the right pass. Read all the things out there about others experiences and judge for yourself how adventurous you are and how much of the planning you're up to and willing to do to save the $$$. If you want it low cost and to not require some due diligence on your part you're out of luck.
Plan well and be prepared for exactly what to expect in planning and physical exertion or spend the difference and let the cruiseline take the profit and the problems you're likely to encounter out of the equasion. If you love Europe and plan to go back regularly as Hank has, you may then be able to better appraise the risks and savings benefits. Going it alone or even with outside vendors is not without risk especially for novices.